The reason for shortening the prayers is travel itself regardless of whether or not one encounters any difficulty.
Allaah and His Messenger (peace and blessings of Allaah be upon him) connected the ruling on this matter (shortening of prayers) to travel. Allaah says (interpretation of the meaning):
“And when you (Muslims) travel in the land, there is no sin on you if you shorten As-Salaah (the prayer) if you fear that the disbelievers may put you in trial (attack you), verily, the disbelievers are ever unto you open enemies”
And the Prophet (peace and blessings of Allaah be upon him) said: “The prayer when travelling is two rak’ahs.” Narrated by al-Nasaa’i, 1420; classed as saheeh by al-Albaani in Saheeh al-Nasaa’i.
The Prophet (peace and blessings of Allaah be upon him) said: “Allaah has waived half the prayer for the traveller.” Narrated by al-Nasaa’i, 2275; classed as hasan by al-Albaani in Saheeh al-Nasaa’i.
This is also indicated by the fact that it is not permissible for the non-traveller to shorten his prayers even if offering the prayer in full is difficult for him, which indicates that this ruling applies only to travel, not to difficulty.
The Standing Committee was asked: If a person travels a long distance by plane, but it only takes two hours or less, can this traveller shorten his prayers and break his fast in Ramadaan or not? Similarly if a person travels by car for two hundred miles or more in two and a half hours, for example, and comes back home in the evening, and he shortens his prayers, is this permissible or is it not permissible unless he faces difficulty and exhaustion when travelling?
Shortening the prayer when travelling the distances mentioned is Sunnah, and breaking the fast in such cases is a concession that is granted to the traveller, whether it takes him a long time or a short time to cross that distance, and whether that causes him hardship or not, because what is assumed is that travelling involves hardship, even if that does not actually happen. This is the bounty and mercy of Allaah towards His slaves.
Fataawa al-Lajnah al-Daa’imah, 8/127.